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Transcript of PHOTOELECTRON SPECTROSCOPY (PES) · PDF file PHOTOELECTRON SPECTROSCOPY (PES) Law of...

  • PHOTOELECTRON SPECTROSCOPY (PES)

  • Law of Photoelectric effect Albert Einstein, Nobel Prize 1921

    Kaiser-Wilhelm-Institut (now Max-Planck- Institut) für Physik Berlin, Germany

    INTRODUCTION

    High-resolution electron spectroscopy Kai Siegbahn, Nobel Prize 1981

    Uppsala university, Sweden

  • Valence band

    EB

    ωh E

    EF

    Evac

    Detector (Ef)

    φ

    Ek

    Discrete core levels

    kinB EE −−= φωh

    ωh BE

    kinE

    φ

    Where, = Binding energy referred to the Fermi level = Photon energy = Work function of spectrometer

    = Kinetic energy of ejected photoelectron

    BASIC CONCEPT

  • ENERGY REFERENCE

    Same level

    e-

    Fermi level

    Spectrometer

    Vacuum level

    φspec.

    .speckinB EE φω −−= h

    sampleBkin EE φω −−= h )( . samplespecsampleBkin EE φφφω −−−−= h

    .specBkin EE φω −−= h

    Core level

    φsample

    Fermi level

    Vacuum level

    EB

    Sample

    ωh

  • BASIC REQUIREMENT OF PES SYSTEM

    1. A monochromatic photon source.

    2. An electron energy analyser (which can disperse the emitted electrons according to their kinetic energy, and thereby measure the flux of emitted electrons of a particular energy.

    3. A high vacuum environment.

  • LIGHT SOURCES

    X-rays

    Al K α 1486 eV

    Mg Kα 1254 eV

    XPS, ESCA

    He-lamp 21.2 eV 40.8 eV

    Ne-lamp 16.8 eV 26.9 eV

    UV

    H-lamp 4 ≤ hv ≤ 12 eV

    UPS

    Synchrotron radiation PES

  • SYNCHROTRON RADIATION

    •Unique feature of Synchrotron radiation �Ultra-bright light, high polarization �Highly directional, collimated beam �Wide spectrum of wavelength, stretching from IR to X-rays

    •Spot size ~1x3 mm2

    •High energy resolution

    Hard X-ray

    Ultraviolet light

    Visible light

    Infrared light

    Radio wave

    Wavelength (meter)

    Atom nucleus

    Atom Protein Cell

    10 eV4000 eV

  • BASIC REQUIREMENT OF PES SYSTEM

    1. A monochromatic photon source.

    2. An electron energy analyser (which can disperse the emitted electrons according to their kinetic energy, and thereby measure the flux of emitted electrons of a particular energy.

    3. A high vacuum environment.

  • ELECTRON ENERGY ANALYSERS

  • ELECTRON ENERGY ANALYSER

    Ek

    Retarding Voltage (Eret.)

    Epass

    Ek-Eret=Epass

  • ENERGY RESOLUTION

    Schematic view of a spherical deflector analyser. Source and exit slit widths s and w. Sphere radii R1 and R2 and the optical radius Rm = (R1+R2)/2.

    Vf

    E

    N(Ek)

    EkinE

    IN

    N(Ek)

    EkinE

    ∆E

    OUT

  • V f

    Epass

    ENERGY RESOLUTION

    )( 2

    1

    1

    2

    R

    R

    R

    R

    e

    E V passf −=

    2

    22 α++=∆

    mmpass R

    w

    R

    s

    E

    E

    broading a giveAnalyser Q

    Energy resolution

    Choose: s, w and Epass� small

  • BASIC REQUIREMENT OF PES SYSTEM

    1. A monochromatic photon source.

    2. An electron energy analyser (which can disperse the emitted electrons according to their kinetic energy, and thereby measure the flux of emitted electrons of a particular energy.

    3. A high vacuum environment.

  • WHY ULTRA HIGH VACUUM (UHV) IS NEEDED ?

    Consider an ideal gas. The number of gas atoms/molecules striking a unit area of a surface per second is given by;

    T

    P

    R

    N

    V

    N RT

    N

    N nRTpVBut

    M

    RT where

    V

    N I

    a

    a

    n

    =⇒==

    == π

    ν ν 8 4

    )( 1

    105.3)( 1

    106.2 8

    4 2 22

    22 24 torrinp

    smMT

    p x

    m

    N inp

    smMT

    p x

    MT

    pR

    R

    N I an 

     

    ≈ 

    

    == π

  • Ex. Assume an atom of radius of ca. 3 Å. Then there is room for ca. 1015

    atom on a cm2.

    With p ≈ 10-6 torr M = 28kg/kmol In ≈ 3x1014 (1/cm2s) T = 300 K

    If every atom sticks to the surface a monolayer is formed in about 3 sec!.

    At p ≈ 10-10 torr a monolayer is formed in about 3x104 seconds≈ 8 hours. Big differences (in sticking probability) between different materials.

    (From experience at 10-10 torr Si, Au etc days, Al, Cu etc. hours, Ti, Mo etc. minutes)

    AN EXAMPLE

  • HOW UHV CAN BE OBTAINED?

    •Good choice of material selected ex. stainless steel

    •Good design of chamber

    •Good pumping systems

    •Long time baking

  • On a larger scale, the sample surface is hit with photon flux focused in a small spot. Electrons emitted by the photon are from ca. 10-100Å of the surface layers depending on electron escape depth in the sample investigated.

    SAMPLE SIZE?

  • Electrons : give a high surface sensitivity Sinceλ = mean free path is small in solids

    λ = average distance an electron travels before it is inelastically scattered, i.e. Suffers energy losses

    λ(Ek) increases monotonically with Ekin (for Ekin ≥ 50 eV)

    SENSITIVITY

  • The flux of electrons of a given energy and momentum decays exponentially with the distance travelled in a material.

    I0 I

    x

    θ

    θcos x

    θλ cos 0

    ⋅ −

    = x

    eII

    λ x

    eII −

    = 0

    Small λ gives high surface sensitivity

    Electrons that have not been inelastically scattered have been generated close to the surface.

    The surface sensitivity can be enhanced by selecting a larger electron emission angle, θ. Normal emission , θ=0o.

    ELECTRON’S FLUX

  • A SIMPLE LAYER ATENUATION MODEL

    d d d

    Io Io Io

    θ

    θλ

    λ

    λλλ

    cos

    0

    0

    ) 3

    () 2

    ()(

    0

    1

    1

    1

    1

    .....)1(

    dtot

    dtot

    ddd

    tot

    e

    II

    e

    II

    eeeII

    −−−

    − =

    − =

    =++++=

    1cos

    cos

    0

    −=

    =

    =

    θλ

    θλ

    d

    bulk

    Surf

    d

    totbulk

    Surf

    e I

    I

    eII

    II d d d

    Io Io Io

    θ

    d

    Surf layer

    Normal emission

    Angle dependent

  • z 0 d

    . 1 ,

    overlayer from )1(

    overlayer no withi.e.

    0

    0

    dthicknessofoverlayerwithe I I

    Thus

    toponoverlayerwhenebyattenuatedissignalsubstrateThe

    ecdzceI

    cdzceI

    d

    Subst

    Overl

    d

    d d

    z

    Overlayer

    z Clean Substrate

    −=

    −=∝

    =∝

    −−

    ∞ −

    λ

    λ

    λλ

    λ

    λ

    λ

  • Photoelectron Spectroscopy (PES) + Angle resolved photoelectron spectroscopy (ARPES)

    Examine core-level hv ≈ 100-4000 eV

    •Elementals annalysis

    •Quantitative analysis

    Thickness

    Location

    Examine valence levels hv ≈ 10-50 eV

    •Surface band structure

    •Density of state (DOS)

    UNIQUE ADVANTAGE FOR PES TECHNIQUES

  • KINETIC ENERGY (eV)

    ELEMENTALS ANALYSIS

    Binding Energy 2s = 117eV Binding Energy 2p = 73 eV

    Binding Energy 2s = 149 eV Binding Energy 2p = 99 eV

  • EFB exhibits chemical shifts (~1-10 eV) depending on chemical surrounding� Information about the surrounding elements can be obtained.

    In solids a specific signal from the atoms in the outermost surface layers can be obtained since they feel a different surrounding (potential) than atoms in the bulk. Surface shiftes < 1 eV.

    Chemical shifts

    Surface shifts

    KINETIC ENERGY (eV)

  • QUANTITATIVE ANALYSIS

  • 4f

    5d

    5p

    5s

    PHOTOIONIZATION CROSS SECTIONS

  • Graphite like carbon

    C 1s SiC

    286 284 282 Binding Energy (eV)

    θe=75o

    θe < 75o

    In t e

    ns it y

    (r e l

    at iv

    e un

    its )

    CHARACTERIZATION

    SiO2

    SiC

    Carbon ?

    )(

    )( ;

    )(

    )( 2 SiCI

    SiOI Si

    SiCI

    gI C ==

  • Emission angle 0 20 40 60

    80

    Si

    C 0

    2

    4

    R at

    io s

    C/Si x 20

    SiO2

    C=0.05 Å

    25.5 Å

    SiC

    )(

    )( ;

    )(

    )( 2 SiCI

    SiOI Si

    SiCI

    gI C ==

    )1(

    )1(

    cos

    coscos

    2 −⋅=

    −⋅⋅=

    eSi

    ox